It is known to use sensors such as a wide range Air Fuel sensor (WRAF) located in internal combustion engine exhaust lines to assess the ratio of fuel and air in exhaust gases. Using information from such sensors allows engine control units to accurately adjust the fuel quantity delivered to meet expected or demanded air/fuel ratio.
One way is that this is done is that the ECU predicts the air to fuel ratio expected at the sensor location based on air entering the engine exhaust gas e.g. re-circulated through the engine as well as fuel injected into the combustion chamber. The expected A/F ratio is then compared to the value from the WRAF sensor and any difference provides data to adjust and correct the (e.g. feedback) control in the fuel supply (e.g. fuel dispensed to the injectors).
The ECU thus relies to a large extent on the accuracy of the WRAF sensor to ensure good control. Sensors are inherently sensitive to specific conditions, such as pressure. In some systems the sensor signal values are adjusted to compensate for pressure variation. Also, to reduce steady state error, the sensor signal value is sometimes self-adjusted in a pure air mixture.
A problem however is that sensors age, and the characteristics change and thus accuracy of sensors change. This is partly due to chemical and mechanical effects in use. It is an object of the invention to overcome these problems.